1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer, direct digital master making machine or similar electrophotographic image forming apparatus. More particularly, the present invention relates to a developing device of the type developing a latent image with a magnet brush and an image forming apparatus using the same.
2. Description of the Background Art
Generally, an electrophotographic image forming apparatus includes an image carrier implemented as a photoconductive drum or a photoconductive belt. A developing device develops a latent image formed on the image carrier to thereby produce a corresponding toner image. It is a common practice with this type of image forming apparatus to use either one of a one-ingredient type developer or toner and a two-ingredient type developer, i.e., a mixture of toner and carrier grains. Development using the two-ingredient type developer features desirable image transferability and desirable developing characteristics against temperature and humidity. The two-ingredient type developer forms brush chains on a developer carrier in a developing zone where the developer carrier faces the image carrier. The toner is fed from the developer on the developer carrier to a latent image formed on the image carrier.
As for the development using the two-ingredient type developer, a decrease in the distance between the image carrier and the developer carrier in the developing zone allows high image density to be easily attained and reduces so-called edge effect. This, however, is apt to cause the trailing edge of a black solid image or that of a halftone solid image to be lost. Let this undesirable phenomenon be referred to as the omission of a trailing edge hereinafter.
The omission of a trailing edge can be reduced if a nip where the magnet brush contacts the surface of the image carrier is reduced in width in the direction of movement of the above surface, as reported in the past. The omission of a tailing edge can be further reduced if the magnet brush is dense at the nip, as also reported in the past.
On the other hand, there is an increasing demand for a high-speed developing device that enhances productivity. A high-speed developing device, however, brings about a problem that the developer scatters or drops. More specifically, assume that the developing device includes a main magnetic pole for causing the developer to rise in the form of a magnet brush and auxiliary magnetic poles respectively positioned upstream and downstream of the main pole in the direction of movement of the developer carrier. The auxiliary poles are opposite in polarity to the main pole and play the role of means for promoting the turn-round of the magnetic lines of force issuing from the main pole. In this configuration, the developer scatters away from the downstream auxiliary pole.
Higher image quality is another target to tackle at the same time as the obviation of the omission of a trailing edge. More specifically, faithful reproduction of thin lines and small dots and reproduction of uniform halftone images with a minimum of granularity are essential with the image forming apparatus. As for the development using the two-ingredient type developer, it has been reported that reducing the grain size of toner and carrier grains is successful to increase resolution and therefore to enhance image quality. However, if the volume mean grain size of carrier grains is 65 xcexcm or below, then the carrier grains are apt to deposit on the image carrier and, in the worst case, cause an image to be lost. This will be referred to as carrier deposition hereinafter. A decrease in the saturation magnetization ratio of the carrier is expected to make the magnet brush dense and soft enough to form an image free from directionality. However, a saturation magnetization ratio of 80 emu/g or below brings about carrier deposition.
More specifically, the carrier grains deposited on the image carrier produce air gaps between the image carrier and a sheet or recording medium, weakening an electric field around the grains. As a result, image portions around the carrier grains are not sufficiently transferred from the image carrier to the sheet and are therefore lost. Further, if such carrier grains are transferred from the image carrier to the sheet, then even the toner grains around the carrier grains are not fixed on the sheet, resulting in defective fixation. The unfixed image would be lost if rubbed off and would contaminate the sheet. Moreover, the carrier grains unfixed on the sheet are apt to move during fixation and cause other image portions to be lost. Conversely, if the carrier grains are left on the image carrier without being transferred to the sheet, then they are apt to scratch the image carrier when removed by a cleaner and cause an image to be partly lost. In addition, when the carrier grains are consumed little by little due to carrier deposition, it is likely that the total amount of the developer and therefore image density becomes short.
The toner and carrier forming a two-ingredient type developer are charged to opposite polarities. For example, when the toner is charged to positive polarity, the carrier is charged to negative polarity. The carrier is therefore apt to deposit on the non-image portion of the image carrier when the toner deposits on the image portion of the same. This carrier deposition is dependent on a potential difference between the non-image portion and the developer carrier. Specifically, even the toner deposits on the non-image portion when the potential difference is small, contaminating the background of the image carrier. The carrier does not deposit on the non-image portion when the potential difference is great.
To obviate carrier deposition, it is necessary to intensity magnetic attraction attracting the carrier present on the developer carrier in the developing zone. Experiments showed that carrier deposition occurred at the downstream end of the nip, i.e., the moment when the developer on the developer carrier left the developing zone. Therefore, the prerequisite is that the auxiliary pole or second pole downstream of the main pole has its magnetic attraction intensified on the image carrier (not on the developer carrier).
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication No. 2000-347506 and Japanese Patent No. 2,517,579.
It is a first object of the present invention to provide a developing device capable of reducing defects including the omission of a trailing edge and preventing a developer from dropping due to a centrifugal force in the event of high-speed development, and an image forming apparatus using the same.
It is a second object of the present invention to provide a developing device capable of reducing the above defects and obviating, even when a carrier with a small grain size is used, carrier deposition to thereby realize faithful reproduction of thin lines and small dots and reproduction of uniform halftone images with a minimum of granularity.
A developing device of the present invention includes a developer carrier, a magnet brush forming device for causing a developer to rise on the developer carrier in the form of a magnet brush with a magnetic pole, and a developer storing member for storing the developer developed a latent image. The magnet brush forming device includes at least a first magnetic pole facing an image carrier carrying the latent image, which is to be developed by the magnet brush, and a second magnetic pole positioned downstream of the first magnetic pole in the direction of rotation of the developer carrier. When the magnet brush formed by the first magnetic pole forms a nip between the magnet brush and the image carrier, flux density in the direction normal to the developer carrier has an attenuation ratio of 40% or above. The second magnetic pole has an upstream half-value point located downstream of a point upstream of and angularly spaced from the edge of said developer storing member by 15xc2x0.